Mosquito Transmitted Diseases

It has been known for over a century that certain mosquitoes can harbor and transmit the microorganisms that cause diseases. These disease pathogens are often viruses which are contracted or “picked up” by the mosquito when it bites (feeds on) an infected host including domestic or wild animals or people. The virus develops and multiplies inside the mosquito. The infected mosquito then transmits the virus through its saliva when it bites another, uninfected host. By passing on the virus, the mosquito acts as a “vector” of the disease.

Yellow fever, WNV and EEE are some examples of mosquito-borne diseases or “arboviruses” (arthropod-borne viruses). Although these viruses have probably been around for centuries, only within the past 100 years has it been determined that they were spread by mosquitoes. Mainly because of organized mosquito control efforts, better sanitation and vaccines, many of these diseases are kept in check and not usually thought of in today’s modern society.

However, the diseases are still prevalent in some parts of the world. For example, malaria, another mosquito-borne disease, kills 2.7 million people each year and infects 300 to 500 million others. The vast majority of the mosquito-borne diseases occur in Africa and the tropics of southeast Asia and South America.

There are 9 arboviruses that have been isolated from mosquitoes collected in Connecticut among which 6 are known to cause human disease.

West Nile virus can cause infection in animals and people. The virus is similar to the St. Louis encephalitis virus and produces similar symptoms. Like EEE, WNV is spread to humans by the bite of infected mosquitoes. A mosquito is infected when it bites a bird that is carrying the virus. The virus is not spread from person to person or directly from birds to people under normal circumstances.

Anyone can become infected with WNV. Most people who are infected with the virus will have no symptoms (80%) or may experience mild illness (20%), before fully recovering. However, a small portion of people (1%), particularly the elderly or persons with compromised immune systems, become seriously ill when infected. In some individuals, the virus affects the central nervous system - the brain and spinal cord. At its most serious, it can cause permanent neurological damage and can be fatal. Onset of symptoms generally occurs 3 to 14 days following the bite of an infected mosquito. Symptoms may range from a slight fever, headache, body aches, rash, nausea and swollen lymph nodes to the rapid onset of severe headache, high fever, stiff neck, disorientation, muscle weakness, coma and, rarely, death. There is no vaccine, or cure for WNV or St. Louis encephalitis though treatment can reduce the severity of the symptoms.

The chance of getting sick from the WNV is very small. In areas where mosquitoes carry the virus, only about one out of 500 mosquitoes are infected. Furthermore, if bitten by an infected mosquito, the chance of a person developing the illness are roughly one in 300. Therefore, the chance of being bitten by an infected mosquito and developing disease symptoms from that bite is very small. This does not mean, however, that people should be complacent. The best way for people to protect themselves in areas where mosquito-borne viruses are present is to take personal protective measures to prevent mosquito bites.

WNV has been isolated from 18 different species of mosquitoes in Connecticut, but 5 species have been implicated as the most important vectors: Culex pipiens, Culex restuans, Culex salinarius, Culiseta melanura and Aedes vexans. Cx. pipiens, Cx. restuans and Cs. melanura principally feed on birds and are largely involved in perpetuating the virus among wild bird populations in nature, while Cx. salinarius and Ae. vexans readily feed on mammals including humans and are believed to be involved in transmission of WNV to horses and humans. Cx. pipiens, a peridomestic species that develops in water with high organic content and is particularly abundant in urban centers, is the most frequently infected mosquito species in the state.

Although WNV has been detected throughout the State of Connecticut, the large majority of infected mosquitoes and human cases have occurred in densely populated urban and suburban communities in lower Fairfield and New Haven Counties and the greater Hartford area which have been identified as regions of the State where the risk of becoming infected is greatest.

The WNV season in Connecticut extends from July through early October but the greatest risk of human infection occurs from August through September.

{Eastern equine encephalitis virus} Eastern equine encephalitis is a rare but serious disease caused by a virus that is spread by adult mosquitoes. On average there are 5 cases each year in the United States. There has never been a documented human case of EEE in Connecticut, but the virus is found in birds and bird-biting mosquitoes that live near wetland habitats along the eastern seaboard from New England to Florida. In some years, high numbers of birds get infected favoring spread to the types of mosquitoes that bite both mammals and birds. These mosquitoes can then infect people and horses. EEE is not spread by people and horses with the disease. The risk of getting EEE is highest from late July through September.

The virus responsible for EEE attacks the central nervous system of its host. Horses are particularly susceptible to the infection and mortality rates approach 100%. Onset is abrupt and horse cases are almost always fatal. Signs of the disease in horses include unsteadiness, erratic behavior, loss of coordination and seizures. There is no effective treatment and death can occur within 48 to 72 hours of the horse's first indications of illness. Horses can and should be inoculated against this disease especially in areas where EEE is known to circulate.

In humans, symptoms of EEE appear from three to 10 days after being bitten by an infected mosquito. Some infected people may not develop illness. For those who become ill, the clinical symptoms may include high fever (103 to 106 degrees F), stiff neck, headache and lack of energy. Inflammation of the brain, encephalitis, is the most dangerous. The disease gets worse quickly and some patients go into a coma within a week. Once symptoms develop, treatment for EEE is supportive and aimed at reducing the severity of the symptoms. As many as one-third of people who get the disease die from it and of those who survive approximately one-half will have permanent neurologic damage. Presently, there is no available vaccine for use in humans.